1. Field of the Invention
The present invention relates to a semiconductor integrated circuit used for a receiving circuit of a wireless communication apparatus.
2. Description of the Related Art
In general, a receiving circuit of a wireless communication apparatus is characterized by converting a high-frequency and weak signal picked up by an antenna into a low-frequency signal and then amplifying it. The receiving circuit comprises an LNA (Low Noise Amplifier), a mixer, and a BPF (Band Pass Filter). The LNA is provided to amplify a weak signal so that the signal will not be buried in noise. The mixer is a circuit that performs frequency conversion. The BPF is used to eliminate an unnecessary signal component such as higher harmonics generated by the mixer or an amplifier, and a signal from an adjacent channel. In the following, description centering on this mixer will be made by taking an example.
FIG. 10 is a schematic configuration diagram showing a conventional semiconductor integrated circuit. In the figure, reference numeral 300 designates a Gilbert cell (see P. R. Gray, R. G. Meyer: Analysis and Design of Analog Integrated Circuits 3rd ed. (1993)) type mixer circuit, a LO signal designates a local signal, an RF signal designates a high-frequency signal, and Vout designates an output signal from the mixer circuit 300.
Next, the operation of the semiconductor integrated circuit will be described.
The mixer circuit 300 receives an RF signal inputted externally from an antenna or the like and an LO signal generated from within the apparatus, multiplies the RF signal and the LO signal together, and then outputs output signals Vout representing their sum and difference in a frequency range.
Since the conventional semiconductor integrated circuit is configured as described above, it has presented a problem in that an output signal gain is not sufficient when the mixer circuit is formed only by a Gilbert cell.
The present invention has been made to solve problems as described above. An object of the present invention is to provide a semiconductor integrated circuit that makes it possible to provide a sufficient gain for an output signal from a mixer circuit and also to extract only a differential component of signals inputted to the mixer circuit, that is, to provide a desirable CMRR (Common Mode Rejection Ratio).
Another object of the present invention is to provide a semiconductor integrated circuit that makes it possible to provide a sufficient gain for an output signal from a mixer circuit, and which is not affected by variations in supply voltage, produces little output distortion, and consumes less power.
According to a first aspect of the present invention, there is provided a semiconductor integrated circuit comprising: a mixer circuit having an upper-stage transistor circuit to which a signal serving as a reference for frequency conversion is inputted and a lower-stage transistor circuit to which another signal to be converted is inputted, wherein a first resistance and a second resistance serving as a load are connected between the upper-stage transistor circuit and supply voltage, and a result of operation on the two input signals performed via these transistor circuits is made to appear as a first signal and a second signal at the first resistance and the second resistance respectively on the basis of the supply voltage; an emitter follower circuit including a first transistor and a second transistor for respectively receiving the first signal and the second signal and outputting a first amplified signal and a second amplified signal that have been subjected to impedance conversion on the basis of the supply voltage; and an operational amplifier circuit for receiving the first amplified signal on an inverting input side via a first input resistance and the second amplified signal on a non-inverting input side via a second input resistance and amplifying these signals to output a result, the operational amplifier circuit including a first feedback resistance for connecting its output side and the inverting input side provided with the first input resistance, and a second feedback resistance for connecting a voltage source and the non-inverting input side provided with the second input resistance.
According to a second aspect of the present invention, there is provided a semiconductor integrated circuit comprising: a mixer circuit having an upper-stage transistor circuit to which a signal serving as a reference for frequency conversion is inputted and a lower-stage transistor circuit to which another signal to be converted is inputted, wherein a first resistance and a second resistance serving as a load are connected between the upper-stage transistor circuit and supply voltage, and a result of operation on the two input signals performed via these transistor circuits is made to appear as a first signal and a second signal at the first resistance and the second resistance respectively on the basis of the supply voltage; an emitter follower circuit including a first transistor and a second transistor for respectively receiving the first signal and the second signal and outputting a first amplified signal and a second amplified signal that have been subjected to impedance conversion on the basis of the supply voltage; and an operational amplifier circuit for receiving the first amplified signal on an inverting input side via an input resistance and the second amplified signal on a non-inverting input side and amplifying these signals to output a result, the operational amplifier circuit including a feedback resistance for connecting its output side and the inverting input side provided with the input resistance.
Here, a capacitance may be connected between the first resistance and the second resistance on the side of the upper-stage transistor circuit.
In addition, the voltage source may be connected to the supply voltage or a ground.
In addition, each of output sides of the first transistor and the second transistor included in the emitter follower circuit may be provided with a resistance for adjustment of output voltage.
Further, a tap resistance for adjustment of output voltage may be provided between the first and second resistances and the supply voltage.
Furthermore, the lower-stage transistor circuit may be constructed by two transistors, whose sizes are made variable.
Preferably, the mixer circuit is of a Gilbert cell type.